1. Field of the Invention
The present invention relates to a powder conveying device, a developing apparatus, a process cartridge, and an image forming apparatus.
2. Description of the Related Art
Image forming apparatuses, such as copiers, facsimile machines, and printers, that employ a toner conveying device have been known. The toner conveying device includes a toner discharging unit that discharges toner out of a toner container, and a conveying tube that connects the interior of a developing device that develops a latent image carried on a latent image carrier, such as a photosensitive drum, and the toner container. The toner discharging unit is activated as necessary, discharges the toner contained in the toner container to the conveying tube, and directly conveys the toner into the developing device through the conveying tube. If the toner container is arranged at a position lower than the position of the developing device in an image forming apparatus including the toner conveying device, it is required to convey the toner, which is the powder that has been passed through the conveying tube, by carrying the toner up against gravity toward the developing device. As a result, the conveying efficiency is degraded, or the toner becomes likely to get jammed inside the conveying tube. Therefore, generally, the toner container is arranged at a position higher than the position of the developing device to convey the toner in the direction of gravity. Japanese Patent Application Laid-Open No. H08-30097, for example, discloses a conventional toner conveying device that performs such toner conveyance in the direction of gravity. The conventional toner conveying device sends toner that has been discharged out of a toner box as a toner container into a conveying tube by a toner discharging device, to the developing device by letting the toner fall with weight thereof.
With the conventional toner conveying device, however, the sectional area of a coil is small with respect to the sectional area of a space inside the conveying tube. Therefore, a part of the section that is not occupied by the section of the coil becomes a clearance through which the toner can pass. Accordingly, when a large amount of toner is discharged at once from the toner container, the toner can flow through the clearance, and can flow into the developing device regardless of rotation of the coil. As a result, a refilling control of toner to the developing device becomes unstable.
Japanese Patent Application Laid-Open No. 2005-24665 discloses a conventional powder conveying device that includes a powder container that contains powder, and a powder conveying tube to guide the powder from the powder container to a destination that is positioned downward relative to the powder container, and that conveys the powder to the destination by passing the powder through the conveying tube. In the conventional powder conveying device, a powder conveying member is provided that is installed inside the conveying tube, and that provides, to the powder, a traveling force to travel downstream in the conveying direction by movement thereof to convey the powder, and a portion that has a higher performance in controlling the powder passage inside the tube than other portions of the powder conveying member in the powder conveying tube is provided at least at one part inside the powder conveying tube.
However, because refilling of the powder is performed when the developing device, which is the destination of the powder, requires the powder according to the required amount, a driving time of a refilling motor becomes long when an image having large image area is processed.
The powder has a small particle diameter to improve image quality. If such powder having a small particle diameter is stirred with air, the volume of the powder increases, and fluidity of the powder increases like liquid. On the other hand, if the powder is left for a long time, the powder enters in even a small space, the volume decreases, and the fluidity is deteriorated.
If images having a large image area is successively printed, toner in a sub-hopper is constantly mixed with a small amount of air present in space, and fluidity increases. If the fluidity increases, friction with the wall of the powder container or with the powder conveying member decreases. Even if driving is stopped, the powder can still be conveyed (flowing) a little. This phenomenon problematically causes accumulation and increase of space at the uppermost stream portion (near a sensor) in the sub-hopper.
In the conventional technologies, if the flowing of the powder in the sub-hopper and the blocking of the powder container occur at the same time, the amount of air at the portion near the sensor exceeds the threshold. As a result, the sensor detects the air to cause the apparatus to erroneously recognize that there is “no toner”.